A conventional boiling water reactor (BWR) includes a reactor core including a plurality of laterally spaced apart nuclear fuel bundles immersed in water. Each of the fuel bundles includes a plurality of fuel rods joined together in a conventional square array, for example, which are completely surrounded on four sides by a correspondingly square fuel or flow channel or tube. The bottom of the flow channel is joined to a conventional conical nosepiece through which water is channeled into the flow channel and upwardly between adjacent ones of the fuel rods and outwardly from an outlet at the top of the flow channel.
Adjacent fuel bundles are laterally spaced apart to define bypass channels between adjacent ones of the flow channels which bypass upwardly water flow around the flow channels. Furthermore, conventional cruciform control rods are also disposed in selected ones of the bypass channels and are selectively translatable upwardly and downwardly for being inserted and withdrawn between adjacent fuel bundles for controlling reactivity of the core fuel.
During operation of the reactor, water is channeled upwardly inside each of the fuel bundles within the flow channels and in parallel flow upwardly between adjacent fuel bundles through the bypass channels. The water inside the fuel bundles is progressively heated and boiled for generating steam which is conventionally used as a power source. The water channeled upwardly through the bypass channels, however, receives less heat from the fuel rods than the water inside the flow channels and does not boil or generate any substantial amount of steam. Accordingly, the bypass water is more effective as a neutron moderator than the water being boiled inside the flow channels and effects neutron flux peaking around the perimeter of the fuel bundles which varies the degree of reactivity and corresponding output power through each of the fuel bundles. In order to obtain a more uniform output power from each of the fuel bundles, a conventional fuel bundle typically includes less enrichment in the fuel rods around the perimeter of each of the fuel bundles which increases the cost of manufacturing the fuel bundles.
Furthermore, the flow channels around each of the fuel bundles are required to ensure that the water is progressively heated and boiled as it flows upwardly along the fuel rods for controlling the resulting steam void fraction. The flow channels, therefore, confine the water flow within each of the flow channels and prevent lateral crossflow of the water between adjacent fuel bundles for ensuring acceptable reactor performance. Furthermore, the spaced apart flow channels also provide the bypass channel therebetween through which the conventional cruciform control rods may be selectively translated for controlling reactivity of the core fuel.
However, the pressure within each of the flow channels varies longitudinally from the bottoms to the tops thereof, and a differential pressure exists across each of the flow channels from the inside thereof to the outside thereof in the bypass channels. The pressure of the boiling water inside the flow channels is greater than the pressure of the water being channeled upwardly through the bypass channels which produces pressure forces acting on the flow channels. Over the course of operation of the reactor core, such pressure forces typically lead to permanent distortion of the flow channels which requires the replacement thereof at periodic intervals.
Conventional flow channels are typically subject to expansion thereof from thermal expansion and from irradiation growth due to the neutrons released during nuclear reactions. For example, conventional flow channels are typically made of conventionally known Zircaloy which expands during operation of the reactor core, which expansion leads to additional permanent distortion of the flow channels during operation. Accordingly, the fuel channels are replaced relatively frequently, for example once per fuel bundle lifetime, which increases the cost of maintaining the nuclear reactor.